[siquick]

How about some premium features for $x a month rather than relying on ads? Asking your most loyal users for $5 a month to hide all ads could get some decent traction. In my experience, users are willing to pay for products they use regularly, but a lot of sites are scared to ask for money.

If you're getting 600k ad impressions per day then your visitor numbers must be fairly high so even a marginal uptake of the premium features would probably beat that $0.1CPM

Look to Mixcloud for inspiration https://www.mixcloud.com/premium/

Also Promoted Posts (i.e. highlighted tracks) may work but you'd need to test if your user base is happy with this as I presume they value your independent voice.